1. Field of Invention
This invention pertains generally to protecting electronic equipment from electromagnetic pulses, and more particularly to a radio frequency (RF) front end designed for immunity to electromagnetic pulses.
2. Description of Related Art
The Graham Commission report, made to U.S. Congress' House Armed Services Committee on Jul. 22, 2004, concluded that a “high-altitude nuclear electromagnetic pulse” is one of the few threats that can hold at risk the continued existence of civil society in the United States. Directed Energy (DE) weapons, including high power Electro-Magnetic Pulse (EMP) weapons and High Power Microwave (HPM) weapons, generate intense pulses of electromagnetic waves that could damage or destroy sensitive electronic circuits. The danger is exacerbated by the fact that the trend towards reduced geometry and voltage renders modern electronics more susceptible to damage from sources of high-power spurious EM radiation, including microwave weapons or nuclear radiation. As an example, a voltage of mere ten volts can punch through the gate of a modern MOS transistor, while voltage of tens of kilovolts or larger can be readily generated by EMP or HPM weapons.
The most vulnerable devices are high density CMOS digital circuits and radio frequency electronics hardware, and, in particular, low noise amplifiers (LNA) of an RF receiver. To meet the stringent speed and noise requirements, these circuits typically use highly scaled transistors with low breakdown voltage. While most components in a system can be protected using Faraday cages, the front-end components are particularly vulnerable, because the antenna provides a direct path for high voltage surge to enter the system. In addition, parasitic or stray capacitances couple energy into circuits providing additional concerns.
Because of the low level of received signal, the receiver circuit is most sensitive to damage from instantaneous voltage surges. Furthermore, while conventional electrostatic discharge (ESD) protection schemes may be able to protect low frequency circuits, presently there are no means to protect high frequency circuits, including wireless and radar front end electronics, from EMP or HPM attacks. For example, the traditional ESD protection approach of using a shunt diode is not applicable at high RF frequencies, since the additional capacitance of the diode will compromise the bandwidth and noise performance of the receiver as illustrated for the low noise amplifier (LNA) front end circuit in FIG. 1.
For directed energy test and evaluation (DE T&E) purposes, sensors are needed to measure electrical fields at high sample rates and wide dynamic range within EMP or HPM beams. In addition, the sensors should be non-interfering, non-intrusive, survivable, and small enough to mount inside targets with limited space. Photonic techniques using optical carriers to interact with electromagnetic fields provide a unique isolation feature between the air interface and the ensuing electronics.
It will also be appreciated that electro-optic probing systems using Pockels effect have been widely demonstrated (see, for example, C. H. Bulmer, “Sensitive, highly linear lithium niobate interferometric waveguide modulator for electromagnetic field sensing,” Appl. Phys. Lett., vol. 53, pp. 2368-2370, 1988, incorporated herein by reference in its entirety, and D. H. Naghski, J. T. Boyd, H. E. Jackson, S. Sriram, S. A. Kingsley, and J. Latess, “An Integrated Photonic Mach-Zehnder Interferometer with No Electrodes for Sensing Electric Fields,” IEEE J. Lightwave Technol., vol. 12, no. 6, June 1994, also incorporated herein by reference in its entirety) and could be considered as a potential solution. However, metals are used as electrodes for the electro-optic modulator or as the transmission line linking a metal antenna to the modulator, and the presence of metal causes two problems. First, metal electrodes and transmission lines can be severely damaged or destroyed in EMP and HPM attacks. Second, in an application as a field probe, these probes must be non-intrusive. This means that they should not effect a significant change in the field pattern when placed in front of, or adjacent to a conventional receiver. For example, non-intrusive survivable sensors are needed to measure high amplitude fields inside a target set (e.g., a missile airframe or a computer system) without altering the EM field inside the structure as if the probe had never been there. Ideally, a problem should be able to capture the entire bandwidth and different polarizations of the field, and be able to withstand extreme power densities ranging from approximately 1000 W/cm2 to 10,000 W/cm2.
Another problem with prior approaches, and one that is particularly onerous when it is used in receivers, is the inherent low sensitivity.